Wind turbines normally comprise a nacelle arranged on top of a tower structure. The nacelle carries a rotor with one or more wind turbine blades, and various components, such as gear arrangement, generator, etc., are normally housed inside the nacelle.
Nacelles often comprise a main frame, a rear frame structure and a nacelle cover. The main frame is a load-carrying structure arranged to be connected to the tower structure via a yaw mechanism, allowing the nacelle to perform rotating movements with respect to the tower structure in order to direct the wind turbine blades towards the incoming wind. The rear frame structure also exhibits load-carrying capabilities, e.g. for carrying some of the components being housed inside the nacelle, and is connected at one end to the main frame. The nacelle cover forms an outer boundary of the nacelle and provides protection against the environment for the components arranged inside the nacelle. The nacelle cover will often not exhibit load-carrying capabilities.
It may sometimes be desirable to apply loads to a rear frame structure of a nacelle which exceed what is expected during normal operation of the wind turbine. For instance, it may be desirable to operate a crane inside the nacelle, e.g. in order to hoist, lower or move heavy components, such as gear arrangement or generator, to, from or inside the nacelle. In order to ensure that the rear frame structure is capable of handling such loads, it is necessary to design the rear frame structure with sufficient strength, e.g. by selecting a material and/or a material thickness providing the desired strength. This may add to the manufacturing costs as well as to the weight of the nacelle. Furthermore, during normal operation of the wind turbine, the strength of the rear frame structure will be significantly above the required strength, i.e. the rear frame structure may be regarded as ‘overdimensioned’ with respect to strength.